You will be familiar with safety flux if you make tiffany-style copper foiled stained glass panels, and if you are, you may be interested in this posting because it can save you some money.

I’ve looked at several manufacturer safety data sheets (MSDS) in an attempt to understand what the important components of solder flux are, especially for safety flux. Some manufacturers very distinctive formulations but there is a common theme – zinc chloride and water. For some manufacturers this is exactly what you are paying for. Overall, it explains why safety flux is advertised as being safe and why you are told it does not contain any acid.

You will find zinc chloride advertised on eBay and you may even be able to get it through other sources – check old chemistry sets, your local stained glass supplier, etc.

Expect to buy zinc chloride as a white powder. Incidentally, you will find that zinc chloride powder will slowly ‘cake’ and get lumpy and hard over the course of time. This is because zinc chloride is hygroscopic, which means it is absorbing moisture from air and all the individual crystals of the powder then stick together. So, keep it in an airtight container.

So, now the key ingredients are identified, how do you make your own?

Keep adding your zinc chloride, a little at a time, into a small plastic bottle containing water. Ordinary tap water is just fine though I might be tempted to use boiled, deionised or even distilled water if my tap water were very hard.

As you progressively add more zinc chloride to the water bottle you will notice the bottle getting warmer. This is because it is an exothermic reaction. It will not get so warm that it will melt a plastic bottle.

You will need a concentrated solution but exactly how concentrated is a matter of opinion. The best way to check you got it right is by testing what you have made – use it so solder some copper-foiled work. If you find it doesn’t work too well, add some more zinc chloride and try again. If you find the extra zinc chloride does not dissolve then you have reached the maximum concentration – a saturated solution.

How hard is that?

If you are a fan of gel fluxes then I have another suggestion – add some carboxymethylcellulose (CMC) to your safety flux solution until you get your favoured consistency. You’ll find I’ve spent a lot of time chattering about buying and making-up CMC ‘gloop’ in this posting.

Finally, a couple of closing remarks about things I recommend you don’t try:

Don’t use ammonium chloride (sal ammoniac) as a substitute for zinc chloride because it’s not nearly as good, it’s acidic and it slowly destroys the tip of your soldering iron. You may have a sal ammoniac block that is supposed to clean your soldering iron tip – lovely to see good old early Victorian chemistry terminology still being used!

Which leads me to suggest you don’t use sal ammoniac to clean your soldering iron tip. Far better to wipe the tip on a damp sponge and much cheaper too!

Don’t use tallow for copper-foiled work because it’s messy and doesn’t work very well.

Happy soldering!

Postscript

Oh dear. I see I missed an opportunity to explain what the flux is actually doing (and not doing) and sneak in a hint of chemistry…

If you have a look in here you will see that zinc chloride is described as being a metallurgical flux. The word “flux” has a particular meaning and for our purposes it means nothing more than “something that aids soldering”. But how is this metallurgical flux aiding us?

What you will learn from the Wikipedia link is that zinc chloride dissolves metal oxide coatings, exposing a clean metal surface. In our context this means that copper oxides that develop on the surface of our copper foil will be removed, leaving a pure copper surface. Copper tarnishes (=oxidises) very quickly so there’s always some copper oxide to be dissolved by the flux.

So, the chemistry must work something like this:

zinc chloride(aq) + copper oxide(s) → copper zinc oxychloride(aq)

The (aq) in those reactions tells you that the named chemical is dissolved in water (Latin: aqua) and the (s) means it is in solid form.

Or, in more general terms we might substitute “M” for “a metal” and describe it as

M + ZnCl2 → MZnOCl2

So, your safety flux works by removing the metal oxide which means it’s nothing more than a “fancy” cleaning product. More to the point, this means your safety flux does not address other contaminants like greasy finger marks and it means it is most definitely not acting like a glue to stick the solder onto the copper foil.

And a final note… I’ve just read that concentrated aqueous solutions of zinc chloride have the interesting property of dissolving starch, silk, and cellulose. We also need to remember that our safety flux is indeed a concentrated aqueous solution of zinc chloride. So, if your clothing or work surfaces are made from natural fibres, such as cotton or wood, watch out!

5 Responses to Make Your Own Safety Flux

Hello! Thanks for your interesting posts about stained glass chemistry! I’ve been wondering about these compounds and their reactions.
I have a question about soldering zinc. I’ve been working on some pieces which are not always square and have been using zinc came to strengthen edges or sometimes internal lines. However, solder doesn’t flow very well on zinc. And using copper patina turns zinc black (what is going on there?). Do you have any thoughts on how I could improve this? Perhaps a stronger acid on the zinc to dissolve the oxide? A different type of solder?

Many books have been written on fluxes and solders over many years and I’m sorry to say I’m no expert and don’t work with zinc. However, a generalisation is that different metals tend to work best with different fluxes and solder combinations. The answer to your problem is therefore to do some research on the Internet and see what other people can tell you. Here are some starting points:

Here and there I encounter references to a suitable solder containing mainly tin with a little zinc and it seems to be linked to the brand name “Roto Metals” (but they’re in the USA). I also find references suggesting lead-free solder (like used for plumbing nowadays).